The life of implanted cardiac pacemakers is to be extended by developing improved electrodes, which will stimulate with low energy, but will require high energy to produce ventricular fibrillation. The eventual objective is to be able to replace non-competitive pacers with fixed-rate units. Polarization effects will be determined by all electrodes. The basic experimental procedure is to implant several electrodes of different types in one dog, and determine acute and chronic stimulation and fibrillation thresholds under a variety of conditions. Preliminary experiments indicate that certain electrodes have the properties just described.